I remember that when I had a max lvl char in BFA, soloing Legion raids was a bit dificult with the wrong class and low ilvl, but doable, whereas WoD and earlier raids were pretty much one-shotable.
Now it looks like all the old content is scaled to be -I think- lvl 60.
Does it mean
1- There is actually no difference in the old content (TBC raid vs BFA raid)?
2- That it’s harder to solo old content?
No. Rules for Mythic and Heroic Dungeons and Raids are different from open world and Normal dungeons. Mythic and Heroic Dungeons and Raids don’t scale.
I have done almost no old raid farming this expansion - or last, for that matter - but raids follow a theoretical level of the expansion.
Everything below BfA is trivial. Legion raids are level 45, IIRC. BfA are level 50. Shadowlands are level 60. There are or were a few annoying mechanics precenting complete soloing of BfA though, I think.